In the field of development of fluorinated elastomers, in later times well defined requirements have experienced a growing strenghtening, that is, the contemporaneous solution of at least one of the aims of group A and one of the aims of Group B as follows:
A--A general improvement in the mixes of the rheological characteristics and the processability, a reduction of the hardness and possibility for an easy extrusion of manufactured articles.
B--A high resistance to steam at high temperature and to inorganic acids, and high chemical stability against recent type of lubricants additioned with aminic compounds, such as the oils of the SF series.
The aims or objects of group A were persued in the past with the help of process-assisting agents of the plasticizing type or by means of various lubricants. The products used so far for this purpose, such as polyesters, polyethylene, pentaerythritol tetrastearate, wax and the likes, etc. did not prove to be suited because they cause an appreciable decay of the typical characteristics of the fluorinated elastomers.
A considerable technical improvement was at a certain point introduced by the use of fluoropolyamides as "process-coadjuvants" for fluoroelastomers based on vinylidene fluoride. It was thus possible, operating according to the procedures described in Italian Patent Publication No. 23572 A/79, to achieve all the objects of point A. Nevertheless, the chemical stability values achieved made these products unsuitable for particular applications, such as for instance oil sealing rings, pipes and gaskets in contact with water and steam at high temperatures.
The achievement of the objects of point B was possible by operating according to the procedures described in U.S. Pat. No. 4,035,565 (Du Pont) and in French Pat. No. 2,386,561 (Daikin). The polymers prepared according to the teachings of the above cited patents differ from the preceding types of fluoroelastomers based on vinylidene fluoride, in as much as they contain carbon-bromine or carbo-iodine bonds. They display a high bulk viscosity further turn out to have processability characteristics inferior to those of the elastomers vulcanizable with bisphenols and are free of carbon-bromine or carbon-iodine bonds. These latter are prepared according to the procedures described, for instance, in U.S. Pat. Nos. 3,876,654--3,712,877 and 4,259,463.
More particularly, in the extrusion processes for the above said polymers, containing carbon-bromine or carbon-iodine bonds, there are required, at equal productivity, higher shearing forces, while there are observed superficial irregularities at lower flow rates (with respect to the fluoroelastomers based on vinylidene fluoride which do not contain vulcanization spots with C-Br and C-1 bonds and are vulcanizable with bisphenols).
With the products obtained according to this teaching it is not possible, however, to achieve all objects of group A. Also as far as the hardness is concerned, the values are always higher than 75 SHORE. Improvements of these characteristics, obviously may be achieved by means of process-coadjuvants of the type of traditional plasticizers and lubricants, but still with the drawbacks already mentioned influencing the properties of the vulcanized products obtained.